The present invention relates to electronic circuits, and, more particularly, to gain control circuits.
Automatic gain control (AGC) circuits are well known and are commonly used for varying the gain of an input signal to provide an output signal having a substantially constant power level. Such circuits may be used in a wide variety of applications, such as radio receivers, bar code readers, audio volume control systems, and the like. In a typical AGC circuit, a variable gain component, such as an amplifier, receives the input signal and produces an output signal. A difference between an amplitude of the output signal and reference value is determined. This difference is integrated and fed back to the variable gain component to adjust the gain of the input signal to maintain the amplitude of the output signal within a predetermined operating window.
An AGC circuit therefore allows an input signal to be adjusted before it is further processed. This is generally necessary because the processing equipment used to recover information from the incoming input signal may work better with a fixed-amplitude signal. Even so, because source signals can vary widely, it may be difficult for an AGC circuit to adjust the gain of the input signal quickly enough (i.e., to converge) to prevent processing errors in certain applications.
One example of an AGC circuit is disclosed in U.S. Pat. No. 4,301,445 to Robinson entitled xe2x80x9cCommunication System and Method Having Wide Dynamic Range Digital Gain Control.xe2x80x9d The AGC circuit applies an input data signal from a logic attenuator to a switchable integrator. The integrator is charged and discharged to periodically develop a voltage signal proportional in magnitude to the amplitude of the data signal from the digital logic attenuator. The magnitude of the voltage signal from the integrator is compared to a voltage reference. A control signal is generated when the voltage signal and voltage reference do not compare, and binary state signals are continuously applied to the digital logic attenuator to effect a change in the amount of attenuation by the digital logic attenuator. The states of the binary state signals are changed during the presence of the control signal and at a time no later than the discharging of the integrator to maintain the magnitude of the data signal at the output of the logic attenuator substantially constant. The stated goals of the patent are to provide a wide dynamic range AGC circuit that is inherently linear and prevents the introduction of inter-modulation and distortion into elements of a system.
Another AGC circuit is disclosed in U.S. Pat. No. 3,931,584 to Motley et al. entitled xe2x80x9cAutomatic Gain Control.xe2x80x9d This AGC includes an analog-to-digital (A/D) converter for sampling an output analog signal from a gain stage including coarse and fine gain controls that provides the amplitude of the samples in digital words characterized by bits including the Most Significant Bit (MSB). The AGC circuit is responsive to the digital characteristics of at least the MSB of the digital words and causes the gain stage to amplify the output analog signal to a substantially constant power level. A stated goal of the patent is to allow amplification of the input signal to a level approaching a maximum level expressible by the digital words and to reduce quantization of noise of the converter by maximizing the number of digital words which are available to express the signal amplitude.
While prior art AGC circuits such as those described above provide certain advantages, there still remain certain applications in which these circuits do not provide adequate response time to input signals with widely varying amplitudes, which may in turn lead to processing errors.
In view of the foregoing background, it is therefore an object of the invention to provide an automatic gain control system and related method that provides improved response times for input signals with widely varying amplitudes.
This and other objects, features, and advantages in accordance with the present invention are provided by an automatic gain control (AGC) system including at least one variable-gain component having a controllable gain over a gain control range, a sensor for sensing an amplitude of a signal from the at least one variable-gain control component and a controller for setting the variable gain component according to coarse and fine gain values. More particularly, the sensor may have an operating window that is smaller than the gain control range of the at least one variable-gain component. The controller may be responsive to the sensor for controlling the at least one variable-gain component according to coarse and fine gain values to set the amplitude of the signal within the operating window of the sensor. The controller may implement at least one coarse gain jump from a current coarse gain value to a new coarse gain value when the sensor indicates the amplitude is outside the operating window. In addition, the controller may further implement movement to a fine gain value when the sensor indicates the amplitude is in the operating window.
The controller may set coarse and fine gain values to substantially center the amplitude of the signal from the at least one variable-gain control component within the operating window of the sensor. Furthermore, a size of the at least one coarse gain jump from the current coarse gain value to the new coarse gain value may be based upon the prior coarse gain value. Each coarse gain jump may be half-way between the current coarse gain value and either a previous lower coarse gain value or a lowest coarse gain value when the sensor indicates the amplitude is at or below a low end of the operating window and half-way between the current coarse gain value and either a previous higher coarse gain value or a highest coarse gain value when the sensor indicates the amplitude is at or above a high end of the operating window, for example. A direction for each jump may be downward when the sensor indicates the amplitude is at a lower end of the gain control range, and upward when the sensor indicates the amplitude is at a higher end of the gain control range. Also, a range of fine gain values may extend at least over a range of spacing between adjacent coarse gain values.
The controller may include a digital processing circuit for generating at least one control signal for the at least one variable-gain component. The digital processing circuit may include an analog-to-digital (A/D) converter for converting an output from the sensor to a digital value, a digital control loop connected to the A/D converter, and an output section for generating at least one control signal for the at least one variable-gain component based upon the digital control loop. Additionally, the at least one variable-gain component may include at least one first variable-gain component having an increasing gain for an increasing control signal and at least one second variable-gain component connected in series with the at least one first variable-gain component and having a decreasing gain for an increasing control signal. The output section of the digital processing circuit may include a digital-to-analog converter.
A method for controlling a signal gain according to the present invention is also provided and may include sensing an amplitude of a signal from at least one variable-gain component having a controllable gain over a gain control range and controlling the gain of the at least one variable-gain component responsive to the sensed amplitude and according to coarse and fine gain values. The controller sets the amplitude within an operating window smaller than the gain control range of the at least one variable-gain component by implementing at least one coarse gain jump from a current coarse gain value to a new coarse gain value when the amplitude is outside the operating window, and implementing movement to a fine gain value when the amplitude is in the operating window.